The present invention relates to an electron beam projection system, such as an electron beam exposure apparatus or an electron microscope, for converging and projecting an electron beam and to magnetic lenses used for the devices or, in particular, to an immersion lens with two magnetic lenses arranged at the sides of a specimen surface on which the electron beam is converged.
The electron microscope currently finds wide applications as a means for observing very small objects. Electron microscopes are of two types, a transmission type and a scanning type. In the transmission electron microscope, in which the electron beam is applied from one side of a specimen and the image of the electron beam transmitted through the specimen is detected, the resolution is determined by the image-forming performance of the magnetic lens functioning as an objective lens. On the other hand, the scanning electron microscope, in which a specimen is scanned by an electron beam converged to a very small spot and the electrons transmitted through or reflected from the specimen are detected, is such that the resolution is determined by the size of the spot of the electron beam which in turn is determined by the image-forming performance of the magnetic lens functioning as a convergent lens.
In recent years, an electron beam exposure apparatus high in resolution and capable of forming a pattern smaller than photolithography has been closely watched as a lithography apparatus for exposing a small pattern for a semiconductor device, etc. An electron beam exposure apparatus can be one of several types. In one type, as in the scanning electron microscope, a pattern is exposed on a resist coated on a substrate by the electron beam converged onto a very small spot. In another type, the electron beam is formed into the desired pattern through a mask on a specimen. In both types, the resolution is determined by the image forming performance of the magnetic lens functioning as a convergent lens. As described above, the resolution of the electron microscope and the electron beam exposure apparatus is determined by the magnetic lens functioning as a convergent lens or an objective lens, and the image forming performance of the magnetic lens must be superior to obtain a high resolution.
The magnetic lens includes a coil, a yoke and pole pieces. The magnetic field created by the coil and the yoke is deformed into the desired shape by the pole pieces. Two or more magnetic lenses can be combined to form a magnetic lens of the desired performance. Generally, in the transmission electron microscope, pole pieces are arranged on the sides of a specimen, and the desired magnetic field is formed above and under the specimen as a magnetic lens. A magnetic lens with such means as pole pieces arranged on both sides of the surface of the specimen (target) for controlling the magnetic field is called the immersion lens. By using the immersion lens, a high resolution is obtained as the desired magnetic field is formed also on the specimen.
In the scanning electron microscope and the electron beam exposure apparatus, in contrast, it is general practice to arrange a magnetic lens only on one side of a specimen, so that the desired magnetic field cannot be easily formed on the specimen surface and a sufficient image-forming performance cannot be obtained. A method conceivable for improving this situation is to employ an immersion type having a superior characteristic in terms of aberration by forming a magnetic field also on the specimen surface. In such a case, the stage for supporting the image cannot be made of a metal but must be made of ceramics.
U.S. Pat. No. 4,544,846 discloses an electron beam projection system comprising an immersion lens having an upper pole piece with a non-zero bore and a lower pole piece including a section with a zero bore.
As described above, the immersion lens used for the transmission electron microscope comprises, in addition to a coil and a yoke, two pole pieces arranged on both sides of the specimen. It is also possible to form an immersion lens by combining two or more magnetic lenses. In this specification, the magnetic field forming means arranged on the incidence side of the electron beam with respect to the specimen is called the first magnetic lens, and the magnetic field forming means arranged on the opposite side the second magnetic lens. The first and the second magnetic lenses of the immersion lens used for the transmission electron microscope are electromagnetic lenses configured of a coil, a yoke and pole pieces. This also applies to the first and second magnetic lenses of the immersion lens disclosed in U.S. Pat. No. 4,544,846, which are electromagnetic lenses.
In the electron beam exposure apparatus, for example, the second magnetic lens is required to generate a strong upward magnetic field of about 0.1 tesla (100 Gausses). The electromagnetic lens capable of generating such a strong magnetic field unavoidably increases in size. The second magnetic lens is arranged under the stage. A large second magnetic lens arranged in such a position reduces the freedom of design of the stage, thereby making it impossible to obtain the desired stage performance.
Another problem is that the electromagnetic lens has the coils thereof supplied with a current. For a large magnetic flux density to be generated, therefore, a large current is supplied which increases the heat generation. The resulting heat distribution occurring in the surrounding area distorts the stage, thereby leading to a lower projection accuracy.